According to German Pat. No. 1 004 516, the lens is for this purpose clamped at high pressure between two cups, so that its position cannot change by itself. To center the lens, the clamping cups are vibrated by ultrasound during the clamping operation in order to convert the static friction between cup and lens into a lower sliding friction. However, this transition occurred sporadically, which often caused damage to the lens due to an undesired material removal.
Furthermore, an attempt has been made to drive the clamping cups during clamping of the lens in opposite directions of rotation. Here too a high risk exists that lens damage will occur, that is, cutting tracks in the form of rings cut into the surface of the lens can hardly be avoided.
German Auslegeschrift No. 21 48 102 suggests to arrange a piezoceramic case vibrator on the elevationally nonchangeable clamping cup, which case vibrator is electrically controlled by a threshold switch such that the clamping cup force drops off when reaching a pregiven pressure, which causes the vibration generator to be turned off. The piezovibrator is used at the same time to test the clamping pressure, to which the vibrational amplitude is regulated. Electronic instabilities are disadvantageous in this arrangement. Furthermore, the vibrator has a not insignificant sensitivity with respect to axial pressure. An initial stress is created during clamping due to the pressure load; a supporting of the vibrator is therefore problematic.
From German Offenlegungsschrift No. 31 39 873 a device is known in which the irregularities of a gear drive are utilized to produce relative movements between lens and clamping cup. A balanced differential is provided as a compensating device between the two parts of a two-part centering spindle and the drive shaft. A hydraulic clamping cylinder is provided for a pressure plate of the upper, axially movable spindle. Due to the high friction of the clamping spindle in its slide bearing, a precise regulating of the clamping pressure is, however, difficult to realize, so that this device can also only be utilized in a limited way.
An important goal of the invention is, while overcoming the disadvantages of the state of the art, to improve the centering of lenses in particular for the cutting operation at the edge through a significantly improved and smooth guiding of the centering spindles, through increased holding forces and exactness of the clamping, without increasing the static force input, so that mechanical damage to the lens are with certainty avoided.
Each centering spindle is according to the invention supported radially and axially in a sleeve, which in turn can be secured through a hydraulic clamping element in the machine frame. The clamping element consists of a sleeve surrounded by a cavity, the sleeve being deformed upon an increase of the pressure in the cavity and being pressed against the guide sleeve of the centering spindle. The centering spindle is in this manner fixed in axial direction and is at the same time exactly aligned with respect to the axes.
Each air bearing can be integrated in the sleeve, which air bearing surrounds the associated guide sleeve of the centering spindle. The upper centering spindle is axially movable and clampable for setting the machine. An adjusting screw acting as a pressure piston makes it possible to adjust the pressure on the guide sleeve of the so-called fixed spindle as needed without changing its axial alignment.
The centering spindle has an outer sleeve which is snugly guided in the air-bearing sleeve and is supported axially and/or radially at the ends. In particular the lower centering spindle, the so-called clamping spindle, is thus centrally axially guided with a high degree of precision and control. The outer sleeve for the guide sleeve of the clamping spindle sits fixedly in the machine housing and is thereby precisely arranged aligned with respect to the outer sleeve for the guide sleeve of the fixed spindle.
A very advantageous further development of the air bearing consists in providing grooves between the housing and at least the lower guide sleeve of the centering spindle, which grooves can be loaded with pressure medium through control openings. The grooves can form axially parallel and/or partial-ring-shaped channels. The inner wall of the guide-sleeve bore is in a preferred manner designed with channels and pockets such that four separate air-cushion fields are created in the upper and lower half, which fields support the clamping spindle with an extremely stable low friction capability during the supply of pressure-air. The air bearing and the therewith connected guiding capability of the clamping spindle and of the membrane pistons makes it possible to keep the aligning forces for aligning of the lens very small and to facilitate delicate adjustments, thus avoiding damage to the surface of the lens during its aligning movement. When the lens has reached its exact aligned position, the cavity surrounding the clamping sleeve is loaded with high pressure, so that the clamping spindle is precisely centrally axially clamped in its position. The supplied pressurized air also helps very much during the machining operation in order to prevent cooling and cutting means or abrasive material from penetrating between the guide sleeve and the clamping spindle.